This invention relates to security devices bearing holographic images, of the type commonly applied to personal identification cards, various types of documents or other substrates, and to methods of making such devices.
As described for example in U.S. Pat. Nos. 5,145,212 and 4,913,504, to protect a credit card or other article or document of value against counterfeiting, a label or like device bearing a relief holographic image may be applied thereto. Holographic images are inherently extremely difficult to replicate effectively; moreover, in the described security device, the relief image is cast on a surface facing the substrate, so as to be inaccessible for taking impressions. Owing to this combination of characteristics, devices of the type described have won very wide acceptance.
Notwithstanding the success of such known holographic image-bearing security devices, it would be desirable to provide enhanced or additional security features in these devices, and to do so in a simple and economical manner, preferably capable of implementation with a minimum of modification in existing types of production lines.